Its one thing to know all about the concept of c++ but its a whole nother beast to be able to use them effectively and with good designs to make a real program. That is probably the hardest part of any programming language.

metaprogramming in C++ is only hard because it's ultimately a really big hack/workaround the C language. One reason why D really caught my eye is that it tries to build metaprogramming into the language from the beginning, the advantage of not being limited to C backwards compatibility... although it doesn't seperate compile-time and run-time functionality very well so it ends up being confused a lot :/

How about a computer with a switch on the back that can only be changed while it is off, with two settings "Fast" and "Safe", which would do as they suggest when your computer is running. Obviously you have to accept an additional end user license agreement before you can start using the "Fast" setting.

IMO there is nothing that i would quickly say was "the hardest thing in C++". But in Software Development I think ensuring integrity and long term maintainability of your work is one of the hardest things. The vast majority of people do a very poor job of this, and this is why ironically I have no shortage in finding contracts.

When working on large software projects you need to ensure you:
- Write code that is simple and easy to follow
- Use logical amounts of documentation to explain your code
- Adhere to your coding standard
- Use unit tests to verify the integrity of your code and protect it from future changes
- Use source control with useful commit messages
- Avoid excessive use of anti-patterns
- Test your work for memory leaks, performance bottlenecks with a profiler
- Avoid refactoring large amounts of work for little functional gain